With this aim of preventing, or at any rate impairing, the forging of a document, security paper provided inside with sheets or yarns of a suitable nature to personalize the document, and thereby enhance its security, are well-known.
More specifically, security yarns are inserted in the paper at the sheet formation stage, so that they remain in the cellulose fibres, and hence any attempt at removing the same implies either tearing the paper or breaking the yarn, which clearly reveals that the document has been tampered with.
Thus, in theory, the provision of a security yarn stands as security for the genuineness of a document.
However, the contemporaneous evolution of counterfeiting techniques has made it necessary to gradually perfect these security yarns, improving their performance, or to put it otherwise, hindering the tampering or copying thereof by counterfeiters. In this sense, yarns have been provided with microprinted texts, which texts can for instance refer to the country of issue, the face value of the document, the date of issue thereof, and so forth, which texts cannot be perceived at first glance but are easily detected using a small magnifying glass or any other suitable means, such as viewing the same against the light, under ultraviolet light and so on.
Techniques have also been developed which expedite the appearance of the yarns on the surface of the paper at regular intervals, to be detected by touch and hence identified by an expert.
Along these lines of evolution, the applicant himself holds Spanish patent of invention application number P 8902252(1), which describes security paper and a method for obtaining the same, which provides for the security yarns to be obtained from an alloy of magnetic material subject to heat treatment comprising a sudden cooling from a starting temperature ranging between 800.degree. and 1500.degree. C. down to a final temperature ranging between -180.degree. and -225.degree. C., thereby to obtain an amorphous solid, devoid of the usual metallic crystalline structure, so that the obtention of this kind of security yarns by potential counterfeiters is almost impossible.